1. Field of the Invention
The present invention relates to a frequency compensation circuit for a sensor used to sense rotating targets, commonly referred to as a gear tooth sensor, which compensates for errors in the operate point of the sensor as a function of the rotational speed of a rotating target.
2. Description of the Prior Art
Rotary position sensors are known to be used in various applications for sensing the angular position of various mechanical devices in various mechanical applications. For example, commonly-owned U.S. Pat. Nos. 4,893,502 and 5,332,956 disclose a rotary position sensor for use in automotive applications for detecting angular movement of a butterfly valve in a throttle body. The rotary position sensor includes a magnetic flux responsive element, such as an analog linear Hall effect device, a magnet, one or more flux concentrators and a shunt ring. The magnet and shunt ring are mounted to rotate with the butterfly valve, while the magnetic flux responsive element and flux concentrators are mounted at a fixed air gap relative to the magnet. With such a configuration, rotation of the butterfly valve causes rotation of the magnet, which, in turn, causes a change in magnetic flux with respect to the magnetic flux responsive element, resulting in a linear voltage being generated that is proportional to the change in magnetic flux and thus proportional to the angular position of the butterfly valve.
Due to the manufacturing tolerances of the components used in such rotary position sensors, such sensors are subject to part-to-part variations. More specifically, the output values of different sensors in response to identical input signals are known to differ due to the manufacturing tolerances of the components used to make the rotary position sensor. In order to compensate for such part-to-part variations, compensation circuits are known. For example, commonly-owned U.S. Pat. No. 6,198,275 discloses a compensation circuit for compensating the sensitivity and offset variations of such sensors. In particular, the compensation circuit disclosed in the ""275 patent includes a processing unit, a test interface and a persistent storage device, such as electrically erasable programmable read-only memory (EEPROM).
Output signals from the rotary position sensor are applied to the processing unit by way of an analog-to-digital converter. The processing unit compares the actual output values with ideal values in order to compensate for the part-to-part variations. The ideal output values for the rotary position sensor are determined at various angular positions of the magnet, relative to the magnetic flux responsive element, to emulate various angular positions of a butterfly valve in a throttle body. The ideal output value at each angular position is detected and stored. The deviations between the actual values and the ideal values are used to determine compensation values for the sensor under test. The compensation values are loaded into the EEPROM and included as part of a sensor module. As such, when the sensor module is installed in, for example, a throttle body, the module provides for automatic compensation of the part-to-part variations. As such, the end user, an automobile manufacturer, need only install the sensor module and need not engage in the time consuming and cumbersome adjustments of such sensors after installation into an automobile.
The compensation circuit disclosed in the ""275 patent provides satisfactory results for rotary position sensors which include linear Hall effect devices for compensating for offset and sensitivity errors of such linear Hall effect devices, for example, over an operating range of less than 360xc2x0. However, such a compensation circuit cannot be used to compensate for other types of errors in other applications. For example, sensors are also used in rotating target applications for sensing when a rotating mechanical member, such as a cam shaft or a crank shaft, crosses a reference point. Such sensors are known as gear tooth sensors or rotating target sensors. In such an application, digital Hall effect devices are known to be used and switch from one logic state to another logic state as a function of the position of the rotating gear teeth or targets relative to a reference point. Unfortunately, errors are known to occur with respect to the switching point of the digital Hall effect device as a function of the RPM of the rotating targets. No systems are known for compensating for such switching point errors as a function of RPM. As such, such sensors are manufactured and shipped to the end user, for example, an automobile manufacturer, which must attempt to compensate for such errors. Thus, there is a need for a compensation circuit for such gear tooth sensors, used in rotating target applications, which provides compensation for the switching point as a function of the RPM of the rotating target.
Briefly, the present invention relates to a compensation circuit for compensating for switching point errors in rotating target or gear tooth sensors. The compensation circuit may include a processing unit and a persistent storage device, such as an electrically erasable programmable read-only memory (EEPROM), for storing compensation values that are used to provide automatic compensation of the rotating target sensor after the sensor module is installed by the end user. The actual and compensation values may be determined and stored as linear functions in the form of mX+b, where X is the frequency of the rotating target, m is the slope and b is the y-intercept. In order to provide compensation, the actual slope value m is multiplied by a compensation value. The y intercept (i.e. b) is selected, for example, to be the maximum speed in a given application. The compensation values (i.e. slope m and intercept b) may be determined on a part-by-part basis by the sensor manufacturer or by utilizing averages, in which case, the calibration values can be determined for the application and pre-programmed by the Hall effect IC manufacturer In operation, the sensor output value is applied to the compensation circuit which processes the actual operate point and provides ideal operate points automatically, thus eliminating the need for the end user to attempt compensate for switching point errors of such gear tooth or rotating target sensors, such as digital Hall effect devices.